The present invention relates to antilock braking systems for motorcycles, having wheel sensors and a monitoring circuit that detects overbraking states from the sensor signals and generates corresponding control signals, and a pressure modulator which adjusts the brake pressures at the two vehicle wheels through the control signals.
Since motorcycles absolutely require sufficient lateral guiding force on curves, brake applications on curves are nearly impossible in the boundary area because the braking forces applied in the lengthwise direction result in a pronounced reduction of the cornering forces of the tires. Of course, controlled brake applications on curves are especially problematical, since every ABS system repeatedly causes brief wheel overbraking states as a result of cyclic pressure application or applies a braking pressure that is at least in the vicinity of the lockup pressure. During these brief phases, the cornering force of the overbraked wheel drops sharply towards zero, so that a vehicle in a tilted position would slide laterally. In addition, the braking forces in the tilted position produce strong erecting moments which the rider must compensate in order to force the inclined position.
Even when the cornering forces would suffice, the pronounced pressure modulations required to correct for wheel overbraking states would cause the rider to make major steering torque changes that occur abruptly at points in time that are not foreseeable, so that the vehicle readily becomes unstable. The antilock braking systems in practical use cycles between the control states "release pressure," "maintain pressure," and "increase pressure" for each wheel.
A pressure release usually occurs whenever the wheel in question exceeds a maximum permissible delay value with sufficiently high slip. This situation occurs at points in time TOein.sub.-- i.
The pressure is maintained (to the extent possible with the system) when the wheel makes the transition from the previous deceleration to the acceleration phase, in other words, moves in the direction of stable slip ranges (occurs at points in time TOhalt.sub.-- i ). The pressure is increased again when the wheel again clearly enters stable slip (points in time TOaus.sub.-- i) , with at least one large pressure jump taking place first, followed by a continuous increase in pressure with a constant gradient (referred to as the "pressure buildup phase" or "reloading phase" in the following), until a wheel acceleration interruption caused by this occurs again and initiates a pressure decrease phase.
When a wheel speed interruption is detected by this method, the wheel has, with a high degree of probability, already exceeded the peak of the .mu.-slip curve, so that the drive provided by the wheel as a function of the road is decreased by an amount that is directly proportional to the degree to which the wheel enters unstable slip. In order to readjust the wheel speed, a reduction by a relatively large pressure value is required so that this feedback effect is counteracted.
While braking on a curve in a sharply inclined position, the rider must compensate for a high steering torque since the wheel contact spot is displaced from the center of the tire toward the edge on the inside of the curve. The magnitude of the steering torque increases linearly with braking force. If pronounced pressure modulations lead to brief, considerable fluctuations in the steering torque curve, the rider will no longer be able to apply a corresponding counter torque at all times. With a poor or excessive steering reaction, the vehicle can very easily become unstable.
German Patent Document 42 04 350.6 describes a method by which wheel overbrakings can be avoided or at least considerably delayed in time. When this is done in such a way that the front wheel is actually kept permanently within slip ranges, which precede the maximum friction value (.mu.-maximum), the vehicle remains stable even when traveling around a curve, with only minor steering torque changes affecting the rider and simultaneously producing nearly optimal deceleration. However, the method suffers from the disadvantage that when the vehicle is sharply inclined there is insufficient reserve cornering force, since the maximum deceleration is exhausted practically permanently. Even with a minor change in the coefficient of friction like that which can occur locally even on surfaces that appear homogeneous, in the final analysis brief wheel overbraking is possible, with the above-mentioned negative accompanying phenomena.
An important problem in antilock braking systems adjusted for curves lies in the reliable recognition of travel around a curve and a sufficiently accurate measurement of the angle of inclination. The methods described in German Patent Document 38 39 520 relate to measurement of the dynamic wheel erecting forces which, in contrast to the statically measured values, increase as the tilt increases. In addition, the tilting moments of a wheel which is traveling in a tilted position are evaluated to measure the degree of tilt, with the tilting moments being determined by different wheel erecting forces on the left and right sides of the wheel. In order additionally to ensure recognition of travel in a tilted position, a specific temporary steering angle pattern is used.
The methods described, however, are subject to high measuring technology expense and do not always provide an unambiguous result.
For example, measurement of the tilting moments requires two force measuring bearings. In addition, the wheel tilting moments depend not only upon the degree of tilt but also on the type and condition of the tires. Furthermore, uncertainty in steering angle pattern recognition is very great since the patterns vary considerably as a function of vehicle speed and rider behavior. Because of the pronounced tread, especially in wide tires, there is no clear relationship between the assumed steering angle and the existing tilted position of the vehicle.
In German Patent Document 38 39 520, as a reaction to recognized travel on a curve, a threshold value for the activation of the antilock braking system is changed with the goal, even at low wheel slip values, of entering the pressure reduction phase.
A measure of this kind, however, will not suffice in many critical situations to ensure sufficient cornering stability. When traveling on extreme curves, the front wheel brake pressure in particular must be kept far below the lockup value, so that the vehicle does not slip sideways. Slip values that clearly indicate overbraking can lead to insufficient cornering forces. The goal of the invention therefore is to provide an antilock braking system for motorcycles in which, once travel on a curve has been clearly recognized, a modified antilocking system is activated that adjusts itself dynamically to the degree of tilt reached.
This and other objects are achieved by the present invention which provides an antilock braking system for a motorcycle having front and rear wheels, comprising wheel sensors that output signals; a monitoring circuit coupled to the wheel sensors to receive the sensor output signals, and which detects overbraking states from the sensor output signals and generates corresponding control signals; a pressure modulator coupled to the monitoring circuit which adjusts brake pressures at the two wheels in response to the control signals; a curve pressure control, coupled to the monitoring circuit, that recognizes travel on curves and controllable regulated braking on curves, said curve pressure control having two acceleration sensors to determine the tilt angle .PHI. of the vehicle and, if this angle exceeds a critical threshold .PHI. krit, causes the monitoring circuit to maintain, via the control signals, the brake pressure at the front wheel before an anticipated lockup pressure is reached.
According to the invention it is proposed to detect travel on a curve and the degree of tilt using two acceleration sensors. Suitable sensors are currently available as semiconductor components that can be used directly with the ABS electronics and involve only minor expense relative to the total cost of the antilock braking system. The primary focus of the invention, however, is not recognition of the tilted position, but reaction to the recognized situation.
The method according to the invention for vehicle stabilization therefore provides comprehensive measures such that, once the tilted position of the vehicle has been detected, the entire antilock braking strategy outside and inside regulated brakings are adjusted to the degree of tilt.
The entry phase is critical for safe braking pressure control. If the front wheel is overbraked once the tilted position exists, it is important for the ABS not to be actively switched on until a wheel locking tendency is recognized, since then only very small cornering forces appear, at least for a short time, which could lead to a fall. Therefore, even before the appearance of the first locking tendencies, an effort must be made to ensure that the wheel brake cylinder pressure on the front wheel does not exceed certain maximum values that must be calculated depending on the degree of tilt recognized.
The strategy followed during active antilock braking to stabilize the vehicle is based upon German Patent Document 42 15 350.6 in which it is proposed to increase the wheel braking pressure in the reloading phases exponentially to the wheel lockup pressure POein stored previously, with the pressure at the beginning of such reloading initially being increased sharply and then increasingly gently with respect to the anticipated lockup pressure POein. The flat pressure curve in the vicinity of POein prevents the wheel with high dynamics from being braked into the unstable slip range. By means of a pressure minimum increase that is required at the end of the reloading phase, POein is reliably achieved once again following a predetermined maximum time.
The curve pressure control according to the invention provides for raising the pressure in the reloading phase exponentially, once again according to German Patent Document 42 15 350.6, but stopping the pressure rise before reaching the previous lockup pressure POein and keeping the pressure constant. The difference between the holding pressure POstop and the lockup pressure POein therefore depends on the braking situation. The greater the tilted position of the vehicle that is reached, the greater the pressure differential that will be chosen, so that a rising cornering force reserve is guaranteed with the increase in tilt.
Therefore, in controlling the pressure on curves, in contrast to the mechanisms in German Patent Document 38 39 520, the present invention provides a component that can predict far in advance, that adjusts itself very flexibly to the existing road and steering conditions and gives the vehicle being braked on a curve sufficient riding stability with very good steadiness.
The advantages of the invention therefore include: reliable recognition of the tilted position of a vehicle; prevention of wheel overbraking state; ensuring sufficient vehicle deceleration; and rapid pressure adjustment in changing situations.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.